Male primates sometimes engage in a behavior known as infanticide, which involves the intentional killing of young offspring by a mature animal of the same species. This complex and often controversial phenomenon has been observed across various primate groups. Scientists study this behavior to understand its underlying reasons.
The Reproductive Advantage Hypothesis
The most widely accepted explanation for male primate infanticide is the reproductive advantage hypothesis, also known as the sexual selection hypothesis. This theory posits that males commit infanticide to accelerate female reproductive readiness, increasing their own mating opportunities and overall reproductive success. Female primates are often unable to reproduce while nursing an infant due to lactational amenorrhea, where lactation suppresses ovulation; an infant’s death can prematurely end this period, making the female fertile and receptive to mating sooner. By eliminating another male’s offspring, an infanticidal male ensures the female’s subsequent offspring carry his genes, maximizing his fitness. This behavior is considered an evolutionary strategy, beneficial only if the male is unrelated to the infant, as males do not kill their own offspring.
Ecological and Social Triggers
While infanticide’s ultimate evolutionary purpose is reproductive advantage, specific environmental and social conditions often trigger its occurrence. A common scenario involves new males taking over a group or a significant change in social hierarchy, creating an opportunity for immigrant males to assert dominance and eliminate previous leaders’ offspring. High population density or resource scarcity can also contribute to infanticide. When resources like food become limited, eliminating infants reduces competition for remaining group members. These conditions serve as immediate triggers for infanticidal behavior.
Female Counter-Strategies
Female primates have developed behaviors and strategies to protect their infants from infanticidal males, such as paternity confusion, where females mate with multiple males, including potential infanticidal males, to obscure their offspring’s true paternity. This makes it less likely any male will kill an infant, as he risks killing his own progeny. Females may also employ concealed ovulation, making their fertility timing less apparent. Females also engage in direct defense, physically preventing aggressive acts or recruiting assistance from other group members by forming alliances with other females or establishing “friendships” with males who might defend their offspring. Mothers may also flee the group with their infant to avoid threats; these counter-strategies aid infant survival where infanticide is a risk.
Species-Specific Manifestations
Infanticide is not universal across all male primate species, and its manifestation varies where it occurs. It is well-documented in species like Hanuman langurs, where it was first observed, often following male group takeovers; baboon species, including chacma and hamadryas baboons, also exhibit male infanticide, often linked to new male immigration. Chimpanzees are another species where male infanticide is observed, though frequency varies between communities. While common in these species, infanticide is rare or absent in others, such as some spider monkey populations. The specific context, frequency, and social structures influence how infanticide manifests across different primate species.